Added 'plugins' option

[nicolagusso]

Checklist

• [ ] run npm run test and npm run benchmark
• [ ] tests and/or benchmarks are included
• [X] documentation is changed or added
• [ ] commit message and code follows the Developer's Certification of Origin and the Code of conduct

[mcollina]

I would also like to see a bump in the Ajv version

[climba03003]

I would also like to see a bump in the Ajv version

ajv bumped in https://github.com/fastify/fastify-response-validation/pull/72

It require two step to be merged

1. add unit test
2. merge code from master

[Uzlopak]

I adapted the changes for my refactor #75

[Eomm]

Superseded by #75